The invention relates to a circuit assembly for doubling the voltage of a battery by means of a charge pump fed by the battery voltage and controlled by a clock generator whose supply voltage is the voltage output by the charge pump.
Portable electronic devices, for example, mobile telephones, obtain their supply voltage from batteries. To save weight and space batteries are used which although being designed smaller and smaller furnish a voltage which is lower than that required for operating the device. So that the voltage needed to power such electronic devices is nevertheless made available, voltage doubler circuits are put to use which may take the form of voltage converters operating on the charge pump principle. This principle of a voltage converter employing a charge pump is described for instance in the German semiconductor circuitry text book xe2x80x9cHalbleiter-Schaltungstechnikxe2x80x9d by U. Tietze and Ch. Schenk, 9th edition, published by Springer Verlag, pages 570/571. With the aid of this voltage converter a voltage can be obtained which is near twice that of the battery voltage. For this purpose the charge pump contains switches which are controlled with the aid of a clock employing signals opposite in phase, the switches in this arrangement being field-effect transistors which are cycled ON and OFF by the clock signals. The way in which such charge pumps work is known as such and thus is not detailed here.
One application of a voltage doubler circuit of the aforementioned kind is to be found in the Texas Instruments integrated circuit TMS37121B. This integrated circuit is put to use in processing analog signals in a transponder. FIG. 2 illustrates the basic circuit diagram of the voltage doubler circuit as employed in this integrated circuit. The clock generator 10 is of coventional configuration and furnishes two non-overlapping clock phases xcfx86 and {overscore (xcfx86)}. It is with these clock phases that CMOS transistors are controlled in the charge pump 12 which function as switches. The charge pump 12 produces from its supply voltage VBat an output voltage VA roughly twice the supply voltage. This output voltage VA can be tapped at the capacitor CL.
The p-channel field-effect transistors in the charge pump 12 can only be switched OFF totally when the clock phases xcfx86 and {overscore (xcfx86)} have attained at least the level of the output voltage VA. This is no problem as long as the circuit is in operation since the clock generator 10 receives as the supply voltage the output voltage VA of the charge pump 12 which is higher than the battery voltage VBat. However, problems are encountered on power up of the circuit since the charge pump 12 does not xe2x80x9cstartxe2x80x9d or, to put it otherwise, it is unable to initially generate any higher output voltage as long as the clock phases xcfx86 and {overscore (xcfx86)} have not attained the necessary level. To get around this problem a diode D is thus made use of which supplies the clock 10 the battery voltage VBat initially as the supply voltage so that the charge pump 12 can start operation. As soon as the charge pump 12 is working it generates at its output an ever-increasing voltage VA, the diode D taking care that the connection between charge pump output and the battery is open-circuited as soon as the output voltage VA exceeds the battery voltage VBat.
As evident, on commencement of operation the supply voltage of the clock generator 10 assumes at the most, the battery voltage VBat less the forward voltage of the diode D. When, however, the battery voltage is already relatively low and also low temperatures of e.g. xe2x88x9240xc2x0 C. exist, the circuit is no longer able to start operating because of the negative temperature coefficient of the diode forward voltage and the threshold voltages of the field-effect transistors in the clock generator, the voltage output by the charge pump is no longer sufficient for this purpose.
The invention is thus based on the objective of providing a circuit assembly of the aforementioned kind which satisfactorily starts operation at low battery voltages and low temperatures.
This and other objects and features are achieved, in accordance with one aspect of the invention, by the source/drain circuit of a field-effect transistor being inserted in the connection between the output of the charge pump and the battery, the field-effect transistor being ON when its gate voltage is smaller than its source voltage. Connected to the gate of this field-effect transistor is the output circuit branch of a current mirror circuit through which a limited small current is derivable from the gate to ground. An auxiliary charge pump is provided which receives its supply voltage from the output of the charge pump and which is likewise controlled by the clock generator, the voltage generated by this auxiliary charge pump being placed on the gate of the field-effect transistor.
By making use of a field-effect transistor in the circuit assembly in accordance with the invention for applying the battery voltage to the output of the charge pump and thus to the supply voltage input of the clock generator, on power up of the circuit the full battery voltage is available as the supply voltage. This avoids the drop in voltage by the diode as employed in prior art. At the same time by making use of the current mirror circuit it is assured that the field-effect transistor is totally ON prior to power up of the circuit while the auxiliary charge pump made use of additionally ensures that this field-effect transistor is OFF as soon as the clock generator 10 is working satisfactorily and furnishes the clock phases for controlling the charge pumps. Accordingly, the circuit assembly is able to start operating satisfactorily even when the battery voltage is low and at very low temperatures.